Prior art charts and targets, such as those illustrated in U.S. Pat. Nos. 5,500,699 and 5,414,479 to Ginsburg, yield variable results depending on the axis of residual astigmatism and higher order aberrations such as vertical coma and other non-rotationally symmetric aberrations. More specifically, prior targets used for measuring contrast sensitivity and spatial frequency response in their preferred embodiment have rows and columns of patches that are presented against a white background. The targets are comprised of patches (˜1.4° of solid viewing angle) of gratings having successive parallel aligned light and dark areas, which parallel aligned light and dark areas have substantially linear character with the contrast levels and/or spatial frequency or size. These gratings are oriented vertically and at ±15° of vertical (left of vertical, vertical and right of vertical) resulting in “three choice test” (three forced choices). The spatial frequencies covered by the targets may range from 0.5 to 64 cycles per degree (CPD), but typically the values have been 1.5, 3, 6, 12 and 18 CPD. The patient is asked to identify the direction of the lines (left of vertical, vertical and right of vertical). The lowest contrast level that each spatial frequency is correctly identified is considered the contrast threshold from which the contrast sensitivity is determined.
Contrast sensitivity testing and spatial frequency response may be performed with a patient's best refraction (spectacles, contacts lenses, etc.) or with no correction in place. The need for taking these measurements with or without refractions has become extremely important in determining the visual outcomes from refractive surgery {radial keratotomy, Photorefractive Keratectomy (PRK) and Laser Assisted Keratomillieusis in Situ (LASIK) and phakic intraocular lenses}.
The problem with the current embodiment of the targets is that they are spatially biased in one direction (usually vertical or near vertical). Refractive errors such as astigmatism and higher order aberrations (coma, trefoil, tetrafoil, . . . ) cause lines to appear darker (higher contrast) in one angular orientation than in the orthogonal orientation where they appear much lighter (lower contrast). In fact, the finding that lines appear darker in one meridian than another is used to determine the axis (orientation) of the astigmatism and higher order aberrations. The Lancaster Wheel and Sunburst target (FIGS. 1A & B) are examples of these tests. In FIG. 1A, the actual appearance of the target is with all radial lines appearing equally dark. With astigmatism and non-rotationally symmetric higher order vertical aberrations the lines appear darker (higher contrast) in the meridian nearest the retina and lighter (lower contrast) in the orthogonal meridian, as shown in FIG. 1B.
Astigmatism simply means the power of the eye is similar to a torus, biconic or toric ellipsoid where there is a strong and a weak power that are 90° apart. In FIG. 1B, the darkest line in the Clock pattern is along the meridian that is 2 o'clock (30° from horizontal). The lightest line is 90° away at 11 o'clock. A patient with astigmatism oriented at 30° would see the sunburst with this appearance. Astigmatism can occur at any axis, so the appearance of the Sunburst or Clock pattern will appear differently depending on the amount and orientation of the astigmatism. Astigmatism is one of the simplest of optical errors and is therefore considered to be Lower Order Aberration optically and can be corrected with spectacles. Higher order aberrations of the eye can occur also (coma trefoil, tetrafoil, . . . ) which are more complex aberrations of the optical system that cannot be corrected with spectacles. They do however, cause the Sunburst pattern to have irregularly darker and lighter spokes. Depending on the exact aberrations of the individual, vertical or near vertical lines will appear differently to each individual causing a difference in the threshold of the contrast of the actual lines seen.
The astigmatic patient with a vertical focal line nearer the retina will see the vertical gratings at a much higher contrast than a person with a horizontal focal line nearer the retina. The result is that the orientations of the astigmatism and non-rotationally higher order aberrations have a direct impact on the results of the contrast sensitivity test and the spatial frequency response. This should not occur. The axis of astigmatism and orientation of symmetric higher order aberrations should have no impact on the contrast sensitivity or spatial frequency response. The method to follow using linear gratings at other orientations, the new “rotationally” symmetric target (“Sinusoidal Bull's Eye” target) and the “fundamental sinusoidal” letters eliminate or reduce any effect of non-rotational refractive errors (low and higher order aberrations).
Aberrations such as astigmatism and other higher order aberrations can make the contrast of the lines vary by large contrast amounts (greater than 50% difference in contrast). Because the appearance of the target contrast to the patient is reduced by the aberrations, the results of the contrast threshold are variable and depend of the orientation of the aberration.